Low Noise Amplifiers (LNAs) are typically used in communication transceivers for the amplification of weak electrical signals. In typical wireless applications, LNAs are generally fabricated in bipolar semiconductor or GaAs MESFET technologies. Two main concerns associated with the design of amplifiers utilized for low noise RF (Radio Frequency) amplification are: (1) the minimization of noise added to the signal by the amplifier; and (2) achieving maximum power transfer between a source producing the electrical signal and the amplifier.
In order to achieve maximum power transfer between the source and the amplifier, the input impedance (Z.sub.IN) of the amplifier must be equal to the complex conjugate of the source output impedance (Z.sub.S), namely, Z.sub.IN =Z.sub.S. In the case where the output impedance of the source is real (Z.sub.S =R.sub.S), then the input impedance of the amplifier must also be real (Z.sub.IN =R.sub.IN). For maximum power transfer between the source and the amplifier, R.sub.IN =R.sub.S. This is commonly referred to as "power-matching".
The noise added to the signal by the amplifier results in a degradation of the signal-to-noise ratio (S/N) at the output of the amplifier. A figure of merit for the amount of noise added by the amplifier is the ratio of the signal-to-noise ratio at the input (S/N).sub.IN to the signal-to-noise ratio at the output of the amplifier (S/N).sub.OUT. This ratio is commonly referred to as the Noise Factor (F) of the amplifier, and is used to calculate the Noise Figure (NF) of the amplifier according to the formula NF=10 log.sub.10 (F), where F=(S/N).sub.IN /(S/N).sub.OUT.
Typically, amplifiers are integrated as monolithic ASICs (Application Specific Integrated Circuits). Maintaining a power-matched condition very accurately over production tolerances of the components that are used in the amplifier has traditionally been difficult. This especially presents a problem when the power-matched amplifier is used at the output of a filter, e.g., crystal filter, SAW filter, etc., whose components typically have a high sensitivity with respect to changes in the source output and termination (amplifier input) impedances. While external matching networks can be added to achieve power-matching, they typically result in noise increase. Accordingly, maintaining a proper power-matched input impedance of the amplifier following the filter over temperature and process spread of the components, while at the same time minimizing the Noise Figure of the amplifier, is critical for the overall performance of the system in which these components are utilized. Generally, an improvement in one area has resulted in a penalty in the other area.
One prior art LNA is the common-gate amplifier. The common-gate amplifier achieves a well defined input impedance without the addition of an external matching network. However, one disadvantage of the common-gate amplifier is that it has a Noise Figure which is generally too high for various wireless and cellular applications. A further disadvantage of the common-gate amplifier is that it has a relatively low current gain, which makes it difficult to achieve a sufficient power gain in the amplifier.
Another prior art LNA is the common-source amplifier. The common-source amplifier typically has a high input impedance and requires an external matching network to achieve power-matching. However, since the common-source amplifier is a single ended circuit, undesirable feedback paths typically result around the amplifier which reduces the gain. The feedback paths may also cause parasitic oscillations and, accordingly, great care must be taken to ensure operational stability of the amplifier. This is difficult in volume production situations. A further disadvantage of the common-source amplifier is in its performance (other than Noise Figure). The external matching network, which is necessary to achieve power-matching, causes distortion in the current signal output by the amplifier, thus degrading its performance.
The present invention is directed toward overcoming one or more of the above-mentioned problems.